Cellulose esters are known to be excellent thermoplastic materials and, accordingly, cellulose esters are utilized in a broad range of applications. The most widely used cellulose esters having good thermoplastic properties are generally cellulose acetate (CA), cellulose acetate propionate (CAP) and cellulose acetate butyrate (CAB). However, other types of cellulose esters can be useful for certain applications. Each of these materials has relatively high melting temperatures (i.e., 150 to 250° C.) and relatively high melt viscosities. Because of this combination of high melting temperature and high melt viscosities, the temperatures needed to melt process these cellulose esters may, in some cases, approach or exceed the decomposition temperature of the cellulose ester. As a result, cellulose esters can degrade during processing which can minimize their usefulness in certain applications.
In order to lower the melt processing temperature, low molecular weight plasticizers may be added prior to or during the melt processing of the cellulose esters. However, such low molecular weight plasticizers may result in certain difficulties. In particular, these plasticizers may often be volatile, have a high water solubility and may not be sufficiently compatible with the cellulose ester.
With regard to plasticizer volatility, in some cases, additional plasticizer must be supplied during compounding and the volatilized plasticizer must be trapped to minimize exposure of personnel to the plasticizer material. In many cases, the plasticizer can migrate to the surface of the finished object where it can plate out, vaporize or otherwise adversely affect the surface properties of a finished article. For example, loss of volatile plasticizer from a finished part can lead to shrinkage, crazing and deterioration of physical properties in a finished cellulose ester article.
Plasticizers can result in additional difficulties as well. For example, cellulose esters are known to have good UV stability. However, many plasticizers that are commonly incorporated into cellulose esters can impart poor UV stability, which can limit the utility of the cellulose ester for certain applications. Additionally, a cellulose ester may not be useful in certain applications if the plasticizer has an affinity for water.
Examples of known plasticizers for cellulose esters include phthalates (e.g. diethyl or dibutyl phthalate), glycerol, triacetin, citrate esters (e.g. triethylcitrate), aliphatic diesters (e.g. dioctyl adipate), phosphates (e.g. triphenyl phosphate), low molecular weight polyethylene glycols and esters of polyethylene glycols. Selection of such prior art plasticizers and the amount of plasticizer is generally based on the compatibility of the plasticizer with the cellulose ester and on the desired properties in the finished article.
In this regard, it should be noted that the compatibility of each plasticizer will generally vary with the particular cellulose ester. For example, dioctyl adipate generally exhibits poor compatibility with cellulose acetates, but good compatibility with most cellulose acetate butyrates.
Moreover, even within a single type of cellulose ester, the compatibility of the plasticizer can change with the degree of substitution (“DS,” i.e., the number of substitutents per anhydroglucose unit). For example, diethyl phthalate (“DEP”) may be used as a plasticizer for cellulose acetate with a DS of 2.5 or below; however, DEP is considered to be a poor plasticizer for cellulose acetate with a DS of from 2.8 to 3.0.
Additionally, as would be appreciated by one of ordinary skill in the art, the prior art plasticizers discussed above may be used to plasticize other polymer compositions, such as polylactic acid and polyvinyl chloride. The difficulties noted above with the use of the prior art plasticizers are also generally present with these polymers.
In accordance with the aforementioned difficulties observed with prior art plasticizers for cellulose esters and other polymers, it would be desirable to develop new plasticizers for cellulose esters that have low volatility, good compatibility and other beneficial properties. It would also be desirable to form polymer compositions from such plasticizers. Still further, it would be desirable to form articles from these polymer compositions.